The Tenth Planet was the second story broadcast in season 4 of Doctor Who. Due to the fact that The Smugglers was the final story in season 3's production block, however, The Tenth Planet was actually the first story produced in this season; from a production standpoint, this was William Hartnell's only contribution to season 4. It also introduced the Cybermen, who would become a longstanding enemy of the Doctor.

The Tenth Planet was a major historical turning point for the Doctor Who series by employing a new concept that would come to be known as regeneration. This plot mechanic has contributed to its long run on television. Likewise, it introduced the idea the Doctor has multiple incarnations and can regenerate to coincide with the departure of an actor playing an incarnation to avoid cold transition from one actor to the next.

This was actor William Hartnell's final story as a regular on the series, as he was asked to leave the role, and the last adventure of the First Doctor. Hartnell returned to reprise the role of the First Doctor one final time in 1972-73's The Three Doctors for the opening of the tenth anniversary season of Doctor Who.

Currently, the fourth and final episode of this serial remains missing from the BBC film archive, with only the regeneration sequence and various other scenes discovered intact. However, the missing episode has been carefully reconstructed in animated format.

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Many years ago, Earth's twin planet, Mondas, drifted away to the edge of space. Its inhabitants grew weak, so their scientists created spare parts for their bodies. Limbs and organs were slowly replaced by metal and plastic. Emotions were removed. The Cybermen were born.

The Doctor's TARDIS lands at the Snowcap space tracking station in Antarctica in December1986. A routine space mission starts going wrong. When the base personnel's suspicions are roused, the Doctor informs them that the space capsule is being affected by the gravitational pull of another planet — a tenth planet in the Solar system.

The loss of a routine space mission and the appearance of that planet in the sky herald the arrival of the Cybermen, who are intent on the destruction of the Earth and the conversion of all humans into Cybermen. Ben and Polly fight to save the world, but it is a battle that may be the Doctor's very last.

The Snowcap space tracking station based on the South Pole is monitoring the launch of the manned Zeus IV atmospheric testing probe as it goes into orbit. The ship is manned by two pilots, Schultz and Williams.

The Doctor, Ben and Polly materialise outside the underground bunker of Snowcap and are soon spotted by men watching the surrounding areas through a periscope. Men are scrambled out to collect the strangers. The Doctor, Ben and Polly willingly enter Snowcap.

Inside Snowcap the Doctor, Polly and Ben are interrogated. General Cutler is sent for, who responds hostilely to the strangers. Cutler decides to put them in the observation room until he has time to deal with them later. Once there, the Doctor notices a calendar on the wall saying it is December 1986. Ben is disappointed he won't be able to meet up with his ship but seems more interested when he discovers that space travel is the norm. This conversation is interrupted by a distress call from the Zeus IV space capsule which has been drawn off-course by an unknown force that drains the energy.

The pilots of Zeus IV explain that it is nowhere near the destination in which they would expect to be. Observing their location, Schultz sees another planet. Williams confirms this and says it seems to lie between Mars and Venus. Their state is also brought into confusion by the fact that the communication to Snowcap is weakened and their fuel is dropping much faster than normal.

Observing the crisis, the Doctor announces that he believes he knows the cause of the problems. He is not listened to but gives a piece of paper to Doctor Barclay. Snowcap sights the new planet before they receive the call from Zeus IV saying their fuel has reached 20% — Barclay says they should attempt to come down.

Schultz and Williams attempt to land, but they have no control over their descent as the new planet is affecting their gravitational pull.

Exasperated, Barclay goes to the Doctor for advice. The Doctor looks at a screen with the planet on it and observes that it is identical to Earth — down to the landmasses. The Doctor asks Barclay to look at the piece of paper that he had given him, and Barclay is astounded to read that the Doctor had predicted this phenomenon before they had even seen it. The Doctor tries to explain how he knew this, saying that millions of years ago there was a twin planet to Earth. Before he can go into any more detail, Cutler storms out, saying, "This is ridiculous." When alone, the Doctor explains to Ben that he fears this means invasion.

Cutler speaks to Mr Wigner, the Secretary General of International Space Command, and informs him of the new planet as well as the strangers. Wigner demands that they be interrogated immediately.

Cutler storms in and demands the truth from the Doctor; he suspects him of being involved in this mystery. He sends men out to search the TARDIS.

The men scramble to the TARDIS. Before they can get out, a mysterious ship lands on the surface of the South Pole. Unable to open the TARDIS, a soldier sends his ally back to get a welder. Whilst alone, he is approached by a robotic creature who knocks him out. When his ally returns, the creature is disguised in the man's clothes. When he turns to reveal himself, he knocks the two men out using human hands.

The Doctor reasserts his belief that there will be visitors to Earth from the mystery planet but is pooh-poohed by Cutler. Cutler speaks to Zeus IV and promises the pilots they will be brought back safely.

Outside Snowcap, the robots disguise themselves in the clothes of their now-dead victims.

At International Space Command, a powerless Wigner is forced to watch the news to learn about the mystery planet as communications have been severed with Snowcap.

While everyone is distracted by their efforts to land Zeus IV safely, the alien invaders enter the base, killing a soldier who tries to disable them on the orders of Cutler. The base personnel and Polly plead with the creatures to allow them to save the lives of the Zeus IV crew, but the aliens proclaim that the lives of the humans are irrelevant to them. They reveal that they are known as Cybermen and were once like human beings. They lived on the planet Mondas, twinned with Earth. As Mondas drifted off into space, cybernetic scientists gradually replaced their bodies with mechanical parts and eliminated the "weaknesses" of illnesses and emotions from their brains. Cutler presses a panic button alerting International Space Control to an emergency. International Space Control gets in touch with Cutler, and the Cybermen order Cutler to inform them that it was a false alarm; Cutler refuses and is rendered unconscious by the touch of a Cyberman. Barclay is approached to do the same and refuses until the Cybermen threaten to destroy the computers that would help Zeus IV to land. Barclay yields, stating it as a fault. Once this has been done, the Cybermen inform Barclay it is impossible for the astronauts to return as Mondas' gravitational force is too close. As the Cybermen begin the process of securing Snowcap, Ben plans their escape. Seeing a gun left on the ground by the dead soldier, he picks it up but is soon spotted by a Cyberman. The gun is removed and bent in the Cyberman's grip, and Ben is removed and imprisoned in a projection room.

The Cybermen allow the Snowcap personnel to contact Zeus IV, but it is too late to save them. The ship is dragged further away from Earth by the new planet and explodes.

The Cybermen demand information on Earth and explain that Mondas' energy reserves are becoming exhausted; they intend to use all of Earth's. It is currently absorbing energy from Earth and will soon destroy it. When Polly asks if they plan to kill all of human kind, the Cybermen reveal they propose to take all humans back to Mondas.

Ben, who has been looking for a weapon, rigs up the projector to blind his Cyberman guard. He baits the Cyberman into the room. After the Cyberman is blinded, he steals the Cyberman's weapon and kills him. He seems to be affected by this death.

Ben sneaks back into the tracking room and hands the cyber-gun to the revived Cutler, who kills the remaining two Cybermen. Cutler contacts Secretary General Wigner at Space Command HQ in Geneva and informs him of the attack. Wigner alerts all military bases and informs them that his son, Lieutenant Terry Cutler, was sent on a mission to rescue the doomed Zeus IV and, in turn, is now trapped in orbit.

As Cutler makes plans to secure Snowcap from further Cyberman invasions, including use of anti-aircraft missiles, the radar technician announces that a fleet of spacecraft have been detected. They are approaching Earth.

Ben and Polly accompany the Doctor to his quarters. Ben checks his pulse and says he seems fine. Polly remarks that he seems "worn out."

Cutler decides it is time to take the fight to the Cybermen. He intends on destroying Mondas with a Z-bomb — a series of nuclear bombs that are placed at strategic points around the world. He rings the International Space Centre. Secretary Wigner at Geneva refuses permission, but Cutler rephrases his question asking if he can do anything he deems suitable in order to destroy the Cybermen. Wigner agrees to this and Cutler takes this as permission to use the bomb. Ben and Polly argue against using the bomb. Ben says the Doctor believed that Mondas will destroy itself anyway when it absorbs too much energy. Cutler does not appreciate this intervention and confines Ben to the same quarters in which the Doctor is resting. Before he leaves, he implores Polly to try to convince Barclay to call off the attack.

When Ben arrives at the quarters, he tries to rouse the Doctor, to no avail. However he soon finds a vent that he begins to loosen with his pen knife.

Dyson and Cutler fuse the bomb. Cutler admits he is scared by what he is about to do but feels that he has no choice. When he returns to the control centre, he is concerned to hear that they have lost contact with Zeus V. Polly talks to Barclay, who admits he is reluctant to launch the bomb, citing the radiation caused by the exploding planet which would cause great loss of life on Earth. Polly suggests they rig the bomb so that it doesn't explode. Their plans are interrupted by Cutler, who explains they are to wait for the Cybermen to land, and then they will destroy them with their own weapons. Barclay and Polly sneak away to find Ben. Barclay explains the route he would need to go through the vent to reach the bomb and tells Ben how to sabotage the rocket to prevent it from reaching Mondas. As Ben leaves, the alarm to signal the arrival of the Cybermen sounds.

The Cybermen land and are immediately mowed down by the hidden soldiers wielding the Cybermen's weapons. The soldiers collect the incapacitated Cybermen's weapons for the next assault.

Ben has reached the bomb and begins the process of disarming the bomb but, at the control centre, Cutler notices Barclay's absence. He goes to investigate, catches Ben while he is sabotaging the rocket and knocks him unconscious. Cutler suspects Barclay of being part of this mutiny but states he needs him for the launch of the rocket.

Later, Barclay starts the countdown to the launch of the Z-bomb. Ben regains consciousness but is disorientated and cannot remember if he sabotaged the rocket before being discovered by Cutler. The countdown approaches zero.

As the countdown to launch the Z-bomb reaches zero, the engines fail on the launch pad. Cutler is enraged. He threatens to kill Ben, Barclay and the Doctor, who has regained consciousness and returned to the tracking room. Driven mad with grief by the apparent death of his son in the Zeus V rescue capsule, Cutler is oblivious to the warnings that another force of Cybermen are attacking the base. He also ignores the Doctor's protestations that Mondas is on the verge of burning up.

As he is about to carry out his threat and kill the Doctor, Cutler is killed by the invading Cybermen. The Doctor tries to mediate with the Cybermen, offering them a home on Earth to replace their broken up planet. The Cybermen say they will not even think about negotiations with a missile pointed at their planet and demand that it be removed before they begin negotiations. As a makeweight, they take Polly as a prisoner onto their ship.

As soon as they arrive at their ship, the Cybermen render Polly unconscious and place her in a large metal chair.

The Doctor takes charge of the control centre and contacts the International Space Centre, who themselves have been taken over by Cybermen, to tell them of the situation. The Cyberman in Geneva contacts the Cyberman at Snowcap and informs him that they must move to their secondary objective, which the Doctor assumes to be the destruction of Earth. The Doctor goes on the loud speaker system and warns all of Snowcap of this.

Down in the pit of the Z-bomb, Ben and Dyson are disarming the bomb. Ben raises the question as to why the Cybermen are not doing this themselves, what with them being naturally stronger. Ben hypothesises that they are intolerant to the radiation in the bomb. Dyson and Ben put this to the test by playing dead and drawing in the Cyberman guard from outside, who immediately is rendered immobile. Ben uses this knowledge to order the return of Polly; however, the Cybermen refuse to relent. The Doctor is now taken to the Cybership. The Cybermen warn Ben that both he and Polly will be killed unless they give up the bomb. Ben looks around for something in the room that is portable and radioactive that they can use to their advantage. With the help of Dyson, he uncouples something from the back of one of the computers he can use for this purpose. They hide behind the door as the Cybermen approach. The Cybermen attempt to gas the two men out. When the two men cannot take any more, they open the door and use a combination of the deactivated Cyberman's gun and the radioactive equipment to deactivate the Cybermen.

On returning to the control base of Snowcap, Barclay shows Ben a device that the Cybermen used to contact each other. Ben uses it randomly to draw the Cybermen in. Whilst they wait, the lights dim as Mondas draws yet more power from Earth. As the Cybermen approach, one of the scientists points to the screen where they see Mondas flash brightly and melt. The Cybermen that were approaching the humans disintegrate as the power source they had drawn from has been retarded. Zeus V returns on screen; its pilot explains that all power has been restored and he can attempt re-entry.

An amazing transformation begins...

Ben runs out to the Cybership to free Polly and a semi-conscious Doctor. The Doctor seems to deliberate as to whether "it's all over" before settling that it's "far from over" and alerting his companions that he must return to the TARDIS immediately. Ben gives him his cloak so he won't "catch his death of cold", but the Doctor gives him an unusually distant reply, concerned with something more important at hand. He says, "Ah, yes. Thank you. It's good. Keep warm," in a very abbreviated way, without a lick of humour to be found. Bearing little attention to Ben or Polly, he even deserts them, rushing off alone.

The weary Doctor returns to the TARDIS and closes the door, trying to take care of a dire situation by himself — his body has worn too thin and he is dying from old age, but very soon, that problem will be resolved. He wants his companions safely away from him when a particular event happens, but they are protesting for him to let them inside anyway. As Ben and Polly hammer on the door, the controls operate of their own accord and the central column begins to rise and fall. The Doctor seizes enough energy to open the door and Ben and Polly finally get in. Barely conscious, the Doctor collapses to the floor. Ben and Polly stand back as the Doctor becomes enveloped in a bright light. As the light dies down, the Doctor's face is different and has the appearance of a younger man.

Space fatigue is hypothesised as the cause of the Zeus IV pilots' tiredness, influenced by Mondas' gravity.

When the Doctor's physical body dies or is on the verge of dying, the Doctor voluntarily undergoes a biological process called regeneration that will recreate his old body at the cellular level into a new and revitalised one.

Episode four is missing from the BBC archives. Only short clips of footage exist, among which are the regeneration sequence itself; the sequence exists thanks to it having been included in an early 1970s edition of Blue Peter that discussed the transition to the new Doctor. The regeneration is one of several sequences from lost episodes (Katarina's death in The Daleks' Master Plan being another) that were inadvertently preserved through their use on Blue Peter episodes that were not wiped.

William Hartnell left Doctor Who due to bad health, but upon leaving had said, "If there is one man in England who can replace me as the Doctor, it's Patrick Troughton!" Troughton took over the role of the Doctor when Hartnell regenerated into Troughton. Hartnell returned in The Three Doctors in 1972 to celebrate the show's tenth anniversary. He died three years later.

The Doctor does not appear in episode three, as William Hartnell was unwell during the week when it was recorded. In-story, the Doctor explains that he is "wearing a bit thin", foreshadowing his regeneration.

Special "computer tape"-style opening and closing title graphics were created for this story by graphic designer Bernard Lodge.

This is the only story to give Cybermen characters individual names (Krang, Jarl etc). After this, Cybermen were only ever named by rank (Cyberleader, Cybercontroller etc).

At the time of the serial's original broadcast, as well as the timeframe of the serial (1986), Pluto was still considered a planet and therefore the story title and dialogue referring to Mondas as the solar system's tenth planet was correct.

The master copy of episode four was lost in 1973 after being lent to the BBC's children's magazine programme Blue Peter for a feature they were compiling about the series. (It is unknown how this episode came to be lost; the episode that was lost after being lent out to Blue Peter was "The Traitors", the fourth episode of The Daleks' Master Plan.)

The writing credit for episode one has Kit Pedler as "Kitt Pedler" and title music is credited to "ByRon Grainer" (and due to all letters being capitalised it appears to be "Byron Grainer", as in the first name "Byron"), when it should read, "By Ron Grainer"; for episode three, Gerry Davis becomes "Gerry Davies".

Sometimes the Cybermen start to talk before their mouths open.

The Cyberman helmets were held together with Gaffatape, which can clearly be seen in this episode.

When the Cybermen are ambushed outside, one of them has part of his headpiece (one of the "jug handles") come loose.

The reprise at the start of episode three is a remount of episode two's cliffhanger, not a telerecording of the final moments of episode two, and the dialogue between Cutler and the technician at the beginning of episode three is slightly different from that which closed episode two.

Twenty years after they helped the Doctor prevent the return of Mondas in their personal timelines, Ben and Polly — who had gone their separate ways in life following their return to Earth on 20 July1966 — reunited on 31 December1986, which was by then their present. Ben believed that numerous governments may have covered up the return of Mondas and the Cyberman invasion of the Snowcap. Even after twenty years, Ben still had nightmares about the Cyberman in the projector room. (PROSE: Mondas Passing)

Despite the televised announcement about the coming of the planet Mondas and the "massive landings" of the Cybermen all over the world, people of Earth remained unaware about extraterrestrial intelligent life until 2006. (TV: The Christmas Invasion)

For the purposes of this list, a "regeneration story" is one in which a regeneration is actually and initially depicted. For this reason The War Games is not included below, even though it is commonly thought of as a "regeneration story". It doesn't actually include a clear scene of regeneration, and the preponderance of stories in other media confirm that the Second Doctor did not regenerate at the end of it. Additionally, immediate post-regeneration stories, like the 2005 Children in Need Special — and ones like Castrovalva, where the regeneration sequence was replayed — are not included.

Regeneration is usually considered a biologic process exclusive to Time Lords and a few other species. However, some consider Regeneration and The Eclipse of the Korven to also be "regeneration stories", though neither describes anything close to a biological process. Korven is a particularly hard one to include in this list, because K9 is said to "regenerate" but is largely physically unchanged.